Every year it seems that there are a couple of strings of tree lights which are partially defective. Half the sting will light up and the other will not. It is not the fuses; they check out. It is not one bulb which keeping half the string in the dark since they say that if a bulb is out, the string will light up. These strings have three wires, presumebly so they work in parallel and not series.
What is the deal here? Maybe they’re just cheap junk and the connections are bad.
That’s it in a nutshell.
There are two sets of bulbs wired in series in each string of lights, and you need to take that “the rest stay lit if one burns out!” thing with a grain of salt. The way those work is the wires that hold up the filaments are somewhat spring-loaded inside the bulb, so if the filament burns out, the legs spring apart and connect an internal bypass. If the shunt doesn’t activate properly, the string will go dead. As nobody’s expecting cheap Christmas lights to have a life-sustaining function, there’s no guarantee that the surviving bulbs will stay lit.
The third wire passes power along to the beginning of the second set of bulbs and to the far end so you can “daisy chain” the strings end to end.
Not a panacea, but they work surprisingly well. I’d say about half the time the rest of the strand lights after jamming it in a socket and clicking two or three times.
Or spring for some led lights.
I bought a string of LED lights this year, and when I put them on the tree and turned them on, half of the string remained dark. Tapping on the loose light got the rest to light up, but LED lights will have the same problems as incandescent ones in terms of one of them causing the rest of the string not to light.
That’s the first time I’ve ever heard of that; if you kept the receipt they normally have a warranty do they not?
Is there such a thing as “quality Christmas lights”? IME, they’ve been semi-disposable junk for decades.
No one else (evidently) will ask. Why are you concerning yourself with this question as of today’s date? Find another problem maybe? There are plenty worth pondering, even if you’ve no others of your own. 
I’m one of those dolts who is rather firmly convinced that if we stopped spending so much time on X nonsense leisure-related and entertainment activities … arguably including Christmas lights and message board participation (depends on how much time you spend on them; some might as well make it a career) … that the world would be a remarkable place (as in remarkably much more pleasant).
Then again, maybe you’re the efficient type (perhaps sadly, I rationalize that I don’t waste as much time as others on the leisure activity that is participating here, if for no other reason than I type 120 wpm).
Fallen – if your only contribution is “this is a stupid thing to concern yourself with,” you’re better off not participating in the thread.
Thanks,
twickster, MPSIMS moderator